In recent years, giant magnetoresistance (GMR) effect devices consisting of ferromagnetic layer/nonmagnetic metal layer/ferromagnetic layer and ferromagnetic spin tunnel junction (MTJ) devices consisting of ferromagnetic layer/insulating layer/ferromagnetic layer have been developed, and are expected for application to new magnetic field sensors and magnetic memories (MRAM).
GMR can bring about giant magnetoresistance effect by controlling magnetization of two ferromagnetic layers mutually parallel or antiparallel by external magnetic field, due to the fact that resistances differ from each other by spin dependent scattering at an interface. On the other hand, MTJ can bring about so-called tunnel magnetoresistance (TMR) effect, such that the magnitudes of tunnel currents in the direction perpendicular to layer surface differ from each other by controlling magnetization of two ferromagnetic layers mutually parallel or antiparallel by external magnetic field (Refer, for example, to T. Miyazaki and N. Tezuka, “Spin polarized tunneling in ferromagnet/insulator/ferromagnet junctions”, (1995), J. Magn. Magn. Mater., L39, p. 1231).
Tunnel magnetoresistance TMR depends upon spin polarizability P at the interface of the ferromagnet and the insulator that are used, and is known to be expressed in general by Equation (1), assuming the spin polarizabilities of two ferromagnets as P1 and P2, respectively.TMR=2P1P2/(1−P1P2)  (1),where spin polarizability P of a ferromagnet has a value 0<P≦1. The highest tunnel magnetoresistance TMR at room temperature so far obtained is about 50% in case of CoFe alloy of P˜0.5.
GMR devices are already in practical use for magnetic heads of hard discs. MTJ devices are presently expected for application to magnetic heads of hard discs and non-volatile magnetic memories (MRAM). In MRAM, “1” and “0” are recorded by controlling two magnetic layers mutually parallel and antiparallel which make up each MTJ device by arranging MTJ devices in matrix, and applying magnetic field by flowing electric current in the interconnection provided separately. The readout is conducted utilizing TMR effect. However, MRAM has such a problem to be solved that, when a device size is made small for large capacity, the electric current needed for magnetization reversal increases due to the increase of demagnetizing field, thereby the power consumption increases.
As a method to solve such a problem, the triple layer structure is proposed in which two magnetic layers combined mutually antiparallel via nonmagnetic metal layer (Synthetic Antiferromagnet, hereinafter referred to as “SyAF”. Refer, for example, to the Japanese Patent Laid Application H9-251621A (1997)). By using such SyAF structure, the magnetic field needed for magnetization reversal is reduced even if a device size is made small, because of decrease of demagnetizing field.
On the other hand, a new spin reversal method not using current magnetic field is recently theoretically proposed, as disclosed by J. C. Slonczewski, “Current-driven excitation of magnetic multilayers”, (1996), J. Magn. Magn. Mater., 159, L1-L7, and was also realized experimentally (Refer, for example, to J. A. Katine, F. J. Albert, R. A. Ruhman, E. B. Myers and D. C. Ralph, “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars”, (2000), Phy. Rev. Lett., 84, pp. 3149-3152).
Said spin reversal method is such that, in a triple layer structure consisting of the first ferromagnetic layer 101/nonmagnetic metal layer 103/the second ferromagnetic layer 105 as its principle is illustrated in FIG. 25, if the electric current flows from the second ferromagnetic layer 103 to the first ferromagnetic layer 101, spin polarized electrons are injected from the first ferromagnetic layer 101 via nonmagnetic metal layer 103 to the second ferromagnetic layer 105, and the spin of the second ferromagnetic layer 105 is reversed. It is called magnetization reversal by spin injection.
In said spin injection magnetization reversal of triple layer structure, if the spin of the first ferromagnetic layer 101 is assumed to be fixed, and if the spin is injected from the first ferromagnetic layer 101 via nonmagnetic metal layer 103, then the injected upward spin (majority spin) gives torque to the spin of the second ferromagnetic layer 105, and arranges said spin in one direction. Therefore, the spins of the first and the second ferromagnetic layer 101 and 105 become parallel. On the other hand, if the electric current flows in reverse direction, and the spin is injected from the second ferromagnetic layer 105 to the first ferromagnetic layer 101, then the downward spin (minority spin) is reflected at the interface between the first ferromagnetic layer 101 and nonmagnetic metal layer 103, the reflected spin gives torque to the spin of the second ferromagnetic layer 105, and tends to arrange said spin in one direction, namely, downward. As a result, the spins of the first and the second ferromagnetic layer 101 and 105 become antiparallel. Consequently, in said spin injection magnetization reversal of triple layer structure, the spins of the first and the second ferromagnetic layer can be made parallel or antiparallel by switching the direction of current.
In recent years, giant magnetoresistance (GMR) effect devices consisting of multiplayer film of ferromagnetic layer/nonmagnetic metal layer, tunnel magnetoresistance effect devices consisting of ferromagnetic layer/insulating layer/ferromagnetic layer, and ferromagnetic spin tunnel junction (MTJ) devices have been drawing attentions as new magnetic field sensors and non-volatile magnetic random access memory (MRAM) devices.
As giant magnetoresistance effect devices, that of CIP (Current In Plane) structure of the type to flow electric current in film plane and that of CPP (Current Perpendicular to the Plane) structure of the type to flow electric current in the direction perpendicular to film plane are known. The principle of a giant magnetoresistance effect device is a spin dependent scattering at the interface of a magnetic and a nonmagnetic layers, and, in general, GMR is larger in a giant magnetoresistance effect device of CPP structure than in that of CIP structure.
In such a giant magnetoresistance effect device, a spin valve type is used which inserts an antiferromagnetic layer near one side of ferromagnetic layer, and fix the spin of the ferromagnetic layer. In case of the spin valve type giant magnetoresistance effect device of CPP structure, since the electric resistance of antiferromagnetic layer is higher than GMR film by about two orders as 200 μΩcm, GMR effect is diluted, and the magnetoresistance of a spin valve type giant magnetoresistance effect device of CPP structure is as small as 1% or lower. Thereby, although a giant magnetoresistance effect device of CIP structure is already in practical use for a play back head of a hard disc, a giant magnetoresistance effect device of CPP structure is not yet practically used.
On the other hand, tunnel magnetoresistance effect devices and MTJ can exhibit so-called tunnel magnetoresistance (TMR) effect at room temperature, such that the magnitudes of tunnel currents in the direction perpendicular to layer surface differ from each other by controlling magnetization of two ferromagnetic layers mutually parallel or antiparallel by the external magnetic field (Refer, for example, to T. Miyazaki and N. Tezuka, “Spin polarized tunneling in ferromagnet/insulator/ferromagnet junctions”, (1995), J. Magn. Magn. Mater., L39, p. 1231).
TMR devices are presently expected for application to magnetic heads of hard discs and non-volatile magnetic random access memories (MRAM). In MRAM, “1” and “0” are recorded by controlling two magnetic layers mutually parallel and antiparallel which make up each MTJ device by arranging MTJ devices in matrix, and applying magnetic field by flowing electric current in the interconnection provided separately. The readout is conducted utilizing TMR effect. However, MRAM has such a problem that, when a device size is made small for high density, the noise accompanying fluctuation of device's quality increases, and the value of TMR is so far insufficient. Therefore, the development of devices having larger TMR is required. As is seen from the Equation (1) above, by using a magnetic material of P=1, infinitely large TMR can be expected. A magnetic material of P=1 is called a half metal.
Such half metals are so far known from band structure calculation as such oxides as Fe3O4, CrO2, (La—Sr) MnO3, Th2MnO7, and Sr2FeMoO6, such half Heusler alloys as NiMnSb and others, and such full Heusler alloys with L21 structure as Co2MnGe, Co2MnSi, and Co2CrAl. For example, it was reported that such conventional full Heusler alloys with L21 structure as Co2MnGe or others can be manufactured by heating a substrate to about 200° C., and making its film thickness to, in general, 25 nm or more by T. Ambrose, J. J. Crebs and G. A. Prinz, “Magnetic properties of single crystal Co2MnGe Heusler alloy films”, (2000), Appl. Phys. Lett., Vol. 87, p. 5463.
It was recently reported that, according to the theoretical calculation of band structure, Co2Fe0.4Cr0.6Al in which a part of Cr as a constituting element of a half metal Co2CrAl is substituted with Fe is also a half metal of L21 type by T. Block, C. Felser, and J. Windeln, “Spin Polarized Tunneling at Room temperature in a Heusler Compound—a non-oxide Materials with a Large Negative Magnetoresistance Effect in Low Magnetic Fields”, Apr. 28, 2002, Intermag Digest, EE01. But its thin film and tunnel junction have not been fabricated. Consequently, like conventional other L21 type compounds, it is not known experimentally at all whether its thin film shows half metal characteristics or large TMR properties.
However, though such a spin injection method is hopeful as a spin reversal method of nano-structure magnetic material in future, the current density required for magnetization reversal by spin injection is as quite high as 107A/cm2 or higher, and this aspect was a practical problem to be solved.
Here, the present inventors discovered that magnetization reversal by spin injection can be obtained at lower current density by flowing electric current from a ferromagnetic layer via a nonmagnetic metal layer or an insulating layer provided separately to the triple layer structure in which two ferromagnetic layers are mutually connected antiparalel via a nonmagnetic metal layer.
Further, the present inventors also discovered that the functional effect similar to that mentioned above can be obtained by using, in place of the above-mentioned triple layer structure, a double layer structure consisting of a ferromagnetic free layer and a nonmagnetic layer, and a triple layer structure consisting of a ferromagnetic free layer, a nonmagnetic layer, and ferromagnetic layer.
Also, though giant magnetoresistance effect devices of CIP structure practically used at present for play back heads of conventional hard discs are being made microfabrication for high record density, insufficiency of signal voltage is predicted as a device is micro-fabricated, and higher quality of giant magnetoresistance effect devices of CPP structure is demanded instead of giant magnetoresistance effect devices of CIP structure, which so far has not been realized.
Except for the above-mentioned half metal Co2CrAl, half metal thin films are fabricated, but it needs for it to heat a substrate at 300° C. or higher, or to anneal at 300° C. or higher after film forming at room temperature, but there have been no reports that the so far fabricated thin film is a half metal. The fabrication of tunnel junction devices using these half metals has been partly attempted, but TMR at room temperature is in all cases unexpectedly low, such that its maximum value is at most between 10 and 20% of the case using Fe3O4. As has been seen, the conventional half metal thin film requires the substrate heating or thermal treatment to attain its structure, and surface roughness increase or oxidation thereby may be considered as one of the causes for no large TMR attained. On the other hand, the thin film differs from bulk material in that it may not show half metal characteristics on the surface, and the half metal characteristics is sensitive to composition and the regularity of atomic alignment. The tunnel junction in particular has difficulty to attain the half metal electronic state at its interface. This is regarded as the cause for large TMR not attained. From the above, there remains a problem that the fabrication of half metal thin film is actually very difficult, and the half metal thin film good enough to be used for various magnetoresistance effect devices has so far not been obtained.
Co2CrAl and Co2Fe0.4Cr0.6Al thin film, which are predicted to be half metal from theoretical calculation of band structure, and the tunnel junction using said thin film have not been fabricated. In magnetic thin film material in general, a thin film and bulk material are particularly different in electronic state on their surface. Therefore, a half metal as bulk material is not guaranteed to be half metal as thin film. Even more, if the theoretical calculation of band structure shows a half metal, a half metal is not guaranteed to be obtained as an actual thin film. This is proved by the fact that the above-mentioned various half metals, so far indicated theoretically, have not been experimentally obtained. Therefore, like conventional full Heusler alloy as L21 type compound, it is totally unknown whether Co2CrAl and Co2Fe0.4Cr0.6Al thin film show experimentally half metal characteristics and large TMR property.
As mentioned above, there are many materials that are theoretically pointed out as a half metal, but none of them made into thin film have not shown half metal characteristics at room temperature. Therefore, there is a problem that large GMR from giant magnetoresistance effect devices of CPP structure at room temperature and large TMR from MTJ devices, which might be expected from half metal, have not been attained.